Method and device for producing a coherent layer of even thickness of liquid or melt on a rotating disk

ABSTRACT

Method and device for continuously producing a coherent layer of a liquid/melt of even thickness by means of centrifugal action along the entire periphery of a rotating disk ( 13, 113 ), liquid/melt being supplied to the disk around its entire axis of rotation from means ( 12, 112 ) and leaving the disk as individual droplets. The means ( 12, 112 ) distributing the liquid/melt onto the disk ( 13, 113 ) has according to the invention essentially the same speed and direction of rotation as the disk, and the liquid/melt undergoes under the influence of centrifugal action circumferentially a spreading and change in velocity across the disk ( 13, 113 ) in contact with the same.

The present invention refers to a method for continuously producing acoherent layer of a liquid/melt of even thickness by means ofcentrifugal action along the entire periphery of a rotating disk, theliquid/melt leaving the disk as individual droplets. The invention alsorefers to a device for accomplishing the method.

Liquid material can mechanically be finely divided into droplets(particles) in several ways. Recently, centrifugal techniques have beenused for the purpose of achieving a smallest possible variation indiameter and of increasing the production capacity when droplets areformed. Originally, these types of device for droplet formation used aperforated wall in a rotating centrifuge in order to generate the actualdroplets. A further development of this method has been to let themanufacturing of particles take place from a rotating disk by means of acentrifugal technique. An example of this is the production of sphericalparticles, where droplets of liquid material by means of centrifugalaction are thrown out from the disk and in a subsequent solidifyingprocess are brought to solidify into particles.

In WO8807414 a droplet formation method and apparatus is shown, a liquidbeing distributed onto disks. A distributing means is adapted todistribute the liquid uniformly and circumferentially on the disks. Thisdistributing means comprises a dosing container which is rotationallyindependent of the disks and from which the liquid is dosed though oneor several dosing openings onto the disks. The liquid then spreadsradially outwards towards cusps and is divided into particles.

However, the diameter of the particles produced in such a way varies toa different extent, and it has thus been most desirable to be able toproduce particles with a smallest possible variation in diameter. InEP-A-0 368 851 attempts have been made to solve this problem byproviding the device with a rotating inner distribution means, fromwhich the liquid/melt is supplied to the disk, and the droplets areformed from the outer periphery of the disk by means of centrifugalaction. When different types of spraying are used it is likewiseimportant that droplets of uniform size are produced.

One problem is to supply the disk with a liquid/melt in a reliable way.It is for example difficult to provide a precise liquid flow by means ofdelivering pumps.

Another problem with centrifugal techniques is that the droplets arespattered or released from the liquid material prematurely. Thisuncontrolled release of liquid from the disk results in that thedroplets/particles formed are not completely round.

Still another problem is to obtain a layer of even thickness at the siteof droplet formation at the periphery of the rapidly rotating disk.

Thus, in order to control the amount of droplet forming material perunit of time and at the same time achieve a smallest possible variationin diameter it is during the manufacturing process very important thatthe liquid material is supplied evenly and continuously. Such an evendosage is especially required when solidifying or gelling sphericalparticles are produced from a liquid material, for example a melt or aliquid containing suspended or dispersed particles by means of arotating disk. An even and controlled supply of liquid or melt isrequired on the disk in order to obtain a smallest possible variation indiameter.

In this connection the term “melt” refers to all types of substances inliquid or semiliquid form which optionally contain suspended ordispersed particles and which can be caused to solidify for example bychanging the temperature, by desiccation or by chemical reactions. Theterm “liquid” refers to all materials in liquid or semi-liquid formwhich allow droplet formation in a device as described above. Thus, theterm “liquid” shall specifically also be considered to comprise melts asdefined above.

The feeding of liquid or melt mainly takes place according to twoprinciples. In EP-A-0 109 224 the feeding takes place by means of fixednozzles which spray the liquid directly onto a disk which is rotating ata high velocity and is provided with grooves at its peripheral edge. Inthe above mentioned EP-A-0 368 851 the feeding is accomplished by meansof a rotating distribution means which uniformly distributes the liquidonto the disk having an angular velocity which differs from that of thedistribution means. Therewith the outer disk rotates rapidly and theinner distribution means rotates slowly.

Both these principles result in a limitation in connection with themoment of dispensing, for example when one or more liquid/melt presenton the rotating disk has a viscosity or other mixing propertiesdifferent from the same liquid/melt at the moment of being fed to thedisk. These differences depend on the shear forces as well as thechanged temperature conditions which the liquid/melt is subjected to onthe rotating disk. This results in that part of the liquid/melt fed tothe disk is transported uncontrolled across the surface of the disk tobe thrown out from the same as too large droplets or particles incomparison with that liquid/melt which has adjusted its rheology to thenew conditions on the disk. The corresponding phenomenon also takesplace with those liquids/melts which are fed to a disk in too large aquantity or when a liquid/melt is fed onto a disk whose surface has fartoo insufficient adhering properties with regard to the liquid/melt.

The purpose of the invention is to produce a method and a device forcontrolled droplet formation, the above-mentioned problems beingavoided.

In order to achieve this purpose the invention has obtained thecharacterizing features of claims 1 and 4, respectively.

In order to explain the invention in more detail illustrativeembodiments thereof will be described below reference being made to theaccompanying drawings in which

FIG. 1 is a half cross sectional view of a first embodiment of a devicefor droplet formation according to the invention with one distributionmeans, the figure including an enlargement of an edge, and

FIG. 2 is a half sectional view of a second embodiment of the device fordroplet formation according to the invention with double distributionmeans.

FIG. 1 shows a device 10 for droplet formation or rotor which isattached to a geometrical axis 11. The device 10 for droplet formationcomprises a distribution means 12 and a disk 13, the disk 13 first beingperpendicularly screwed on the threaded axis 11 and the distributionmeans 12 in a corresponding way then being axially threaded on the disk13.

In an upper part of the distribution means 12 a cavity 14 is providedaround the entire axis, the cavity in that way obtaining acircumferential annular aperture upwards. The inner wall of the cavityis provided with a conical surface 15 which can be designed in differentways. Spokes 16 are also substantially vertically arranged on the innerwall at equal spacing from each other. These are radially connected withthe outer wall 17 of the cavity, which extends downwards and out towardsthe disk. A notch 18 in the form of a rounded edge down towards the disk13 is circumferentially provided below the spokes along the outside ofthe cavity.

The distribution means 12 and the disk 13 are circumferentially providedwith a cooperating notched deviation 21 in the form of a labyrinth whichfor example can be designed as opposite ring-shaped ridges and groovesin the underside of a lower projecting part of the distribution means 12and the upper side of the disk 13 in such a way that when thedistribution means and the disk are fastened a space 19 is formedbetween them. The size of this space is determined by means of a washerwhich is exchangeable and arranged below the distribution means 12 atthe threaded part of the disk 13.

The edges 23 of the ring-shaped notched deviations, which are formed inthe counter sinking, are not right angled but beveled with an angle αoutwards the periphery of the disk as well as inwards the axis of thedevice for droplet formation. The purpose of the notch 18 is to achievea film which is retained on the disk, the circumferential supply ofmaterial being completely even.

When exercising the method according to the invention the distributionmeans 12 of the device 10 for droplet formation is supplied in thecavity 14 with that liquid/melt which is used for the production ofparticles (droplets). The liquid/melt is homogenous with reference toits contents and is continuously supplied by means of for example one orseveral stationary nozzles (not shown) while the device for dropletformation is rotated. The speed of rotation depends on the size of thedisk 13 as well as the properties of the material supplied andpreferably amounts to less than 10000 rpm.

All surfaces which are supposed to come into contact with a liquid/meltare adapted to its properties in order to achieve an effective wettingby the liquid/melt. Thus, with for example a hydrophobic liquid/melt thecontact surfaces of the device for droplet formation also arehydrophobic.

By means of the centrifugal force obtained during the rotation theliquid/melt is transported from the conical notch 15 towards the outerwall 17 of the cavity 14. The conical notch on the inner wall of thecavity promotes an increased velocity and stirring of the liquid/melt,which is important especially when the liquid/melt consists of differentcomponents with different rheological properties. By the outer wall 17being angled outwards/downwards the liquid/melt is transported by meansof the centrifugal force along the wall towards the disk 13 and past thespokes 16 which also can be aligned in such a way that the best possibleadjustment of the flow is achieved during the rotation. In thisconnection the circumferentially arranged notch 18 assists in retainingan even flow of the liquid/melt during its motion from the spokes 16down towards the upper side of the disk 13 to be spread out thereonaround the entire axis of rotation.

Thus, the liquid/melt reaches the upper side of the disk 13 around itsentire periphery under the notch 18 and inwardly of the deviation notch21 at a straight part of the space 19 at 22. By means of centrifugalaction the liquid/melt is allowed to spread outwardly towards theperiphery of the disk and is during this transfer forced to pass the“arresting” notched deviation 21 with its space of equal thickness, acircumferential adjustment in the distribution of material beingobtained. After the passage through the circumferential notcheddeviation in the disk the liquid/melt consists of a layer of eventhickness which circumferentially spreads to the periphery of the disk,from which the actual droplets are generated.

According to the invention the liquid/melt is formed into a continuouslayer of even thickness by passing the same through the circumferentialnotched deviation. The function of the notched deviation is tocounterbalance the variable distribution of liquid/melt fed to the diskby providing velocity changes of spreading both radially andtangentially. In this way a liquid/melt of even and continuouslydecreasing film thickness is obtained circumferentially, which is thusforced to be retained on the disk during its radial spreading towardsthe periphery of the disk without any further control and/or restrictionfrom above.

A layer of even thickness of liquid/melt is thus according to theinvention achieved by means of the reduced influence of shear forces,viscosity changes and other dispensing properties in connection with thefeeding onto the disk. The layer spreads radially on the rotating diskin contact with the same all the time and with a continuously decreasingthickness out towards its periphery and up to the site where dropletsare formed circumferentially.

Preferably, the device for droplet formation is according to theinvention constructed with several “stories”. Such a construction of asecond embodiment is shown in FIG. 2. Also in this embodiment of theinvention a device 110 for droplet formation is attached to an axis 111.The device 110 for droplet formation comprises distribution means 112Aand 112B as well as a disk 113, the disk 113 first being perpendicularlyscrewed on the threaded axis 11 and successively the distribution means112A then being axially threaded on the disk 113 and the distributionmeans 112B being threaded on the distribution means 112A.

In comparison with the device 10 for droplet formation shown in FIG. 1the distribution means 12 corresponds to the distribution means 112A.The lower projecting part of this distribution means is, however,extended in such a way that its upper side corresponds to the upper sideof the disk 13. A further distribution means 112B is screwed on theupper part of the distribution means 112A. The remaining components ofthe device 110 for droplet formation correspond to those in FIG. 1, andthis embodiment of the invention thus includes for example cavities,spokes, notches, spaces, washers, and edges. Additional “stories” can inthe corresponding way be arranged on a device for droplet formationaccording to the invention.

By the device 10 according to the invention for droplet formation beingprovided with a distribution means, by which the liquid/melt is suppliedto the disk 13, an adjustment of the liquid/melt is achievedcircumferentially. At the same time the liquid/melt is enforced acontinuous and controlled spreading and change in velocity across thedisk.

This velocity can be varied in dependence of the specific application inwhich the liquid/melt shall be used. The velocity of spreading acrossthe disk can of course be varied by changing the speed of rotation,which normally occurs when the droplet diameter is changed.

The flow of liquid/melt, and thus its velocity of spreading across thedisk, can be adjusted by changing the thickness of the washer 20. Bythis the distance between the distribution means 12 and the disk 13 inthe notched deviation 21—and thus the size of the space 19 also—will bechanged correspondingly, a throttling of the flow of the liquid/meltbeing obtained. A distance in the notched deviation of 0.01-10 mm,preferably 0.1-2 mm, has turned out to be suitable for mostapplications.

Furthermore, the design of the edges 23 has turned out to be of utmostimportance in order to achieve a circumferentially continuous evendistribution of the liquid/melt by controlling its velocity of spreadingacross the disk. In this connection the angle α of the edges should bechosen to be between 0 and 60°, preferably between 1 and 5°.

Thus, by adjusting the distance between the distribution means 12 andthe disk 13 as well as the angle α the device for droplet formationaccording to the invention can be adapted to different liquids/melts insuch a way that a spreading in all instances results in a film with anevenly decreasing thickness towards the periphery of the disk, dropletsof minimal variation in diameter being formed.

The film of the liquid/melt is under controlled conditions transportedto a specific site, from which discrete and individual drops (particles)are formed. The velocity of the film across the disk is reduced incomparison with centrifugal techniques according to the state of theart, which work at corresponding angular velocities. By the totallycontrolled and restricted spreading and change in velocity theliquid/melt can according to the invention be retained on the disk andin contact with the same all the time.

The size of the droplets which can be produced by means of a device fordroplet formation according to the invention can be varied within such alarge interval as 1-3000 μm. However, the size normally lies within therange of 10-500 μm. A number of experiments with different liquids/meltshas shown that a more uniform particle diameter is achieved with alesser dependence on variations in feeding capacity.

A comparative example is given below, in which particles producedaccording to WO8807414 are compared with those produced according to thepresent invention. In this connection several experiments were carriedout, in which the agarose particles produced had an expected diameter of100 μm. The statistical characteristics from these experiments aresummarized below in table 1.

TABLE 1 Charcateristics WO8807414 The present invention Particle diam.(μm) min 25.08 54.36 41.60 80.31 78.55 max 256.16 206.18 138.80 144.08132.28 Median value (μm) 169.52 130.84 114.00 100.73 98.00 Mean (μm)168.11 131.17 112.70 102.06 98.04 Std. dev. (μm) 27.85 28.43 13.50 10.908.66

As can be seen in Table 1 from the maximum and minimum particlediameters, the dispersion in particle diameter is much larger whenparticles are produced according to the state of the art (WO8807414)than when produced according to the present invention. The difference inthe method used is also reflected by the significantly smaller standarddeviation of the inventive particles.

Similar results were also obtained with 100 μm particles made of agarand cellulose. Thus, data are given which represent statisticallysignificant differences between particles produced according to thestate of the art (WO8807414) and those produced according to the presentinvention.

1. In a method for continuously producing a coherent layer of aliquid/melt of even thickness by means of centrifugal action along theentire periphery of a rotating disk (13, 113), the improvementcomprising supplying the liquid/melt to the disk around its entire axisof rotation from a distributing means (12, 112) and exiting the disk asindividual droplets, wherein the distributing means (12, 112) fordistributing the liquid/melt onto the disk (13, 113) is rotating atessentially the same speed and direction of rotation as the disk, andthe liquid/melt under the influence of centrifugal actioncircumferentially undergoes a spreading and change in velocity acrossthe disk (13, 113) in contact with the same.
 2. The method of claim 1,further comprising mechanically stirring the liquid/melt in thedistribution means (12, 112).
 3. The method of claim 1, wherein anadjustment of the flow is produced in the distribution means (12, 112)at the disk (13, 113).
 4. The method of claim 1, wherein the spreadingand change in velocity of the liquid/melt across the disk (13, 113) isproduced by forcing the liquid/melt to pass an arresting notcheddeviation/labyrinth (21) between the disk (13, 113) and the distributionmeans (12, 112).
 5. The method of claim 4, wherein the passage throughthe arresting notched deviation/labyrinth (21) is adjustable. 6.Particles and droplets produced according to the method of claim 1; saidparticles and droplets having a diameter median value between 98 and 114μm and a mean diameter between 98.04 and 112.7 μm with a standarddeviation between 8.66 and 13.50.
 7. A device for continuously producinga layer of a liquid/melt of even thickness by means of centrifugalaction along the entire periphery of a rotating disk (13, 113)comprising a rotor (10, 110) and a disk (13, 113), each having an upperside and underside, a distribution means (12, 112), which is arrangedadjacent to the disk, which distributes the liquid/melt onto the disk(13, 113), a cavity (14, 114A, 114B) around the entire disk around theaxis of the rotor, and a space (19) between the disk and thedistribution means, said space including an arresting notcheddeviation/labyrinth (21) radially outside the cavity and inside theperiphery of the disk.
 8. The device of claim 7, wherein the space (19)is zig-zag-shaped and consists of ring-shaped ridges and grooves whichare made in the underside of the distribution means (12, 112) and theupper side of the disk (13, 113), respectively.
 9. The device of claim8, further comprising an exchangeable washer (20) interposed between thedistribution means (12, 112) and the disk (13, 113) to allow adjustingof the height of the zig-zag-shaped space (19).
 10. The device of claim8, wherein the edges (23) in the zig-zag-shaped space (19) are beveledwith an angle α.
 11. The device of claim 10, wherein the angle α isbetween 0 and 60°.
 12. The device of claim 11, wherein the angle α isbetween 1° and 5°.
 13. The device of claim 7, further comprising aplurality of wings (15) arranged at equal spacing from each other in thecavity (14) towards its inner wall.
 14. The device of claim 7, whereinthe distribution means further comprises a second distribution means(112B) arranged on a first distribution means (112A), a lower projectingpart of the first distribution means including a disk (113) for thesecond distribution means.